Router types

The following are OSPF router types and their positions in the AS:

• Internal router—All interfaces on an internal router belong to one OSPF area.

• Area Border Router (ABR)—An ABR belongs to more than two areas, one of which must be the backbone area. It connects the backbone area to a non-backbone area. The connection between an ABR and the backbone area can be physical or logical.

• Backbone router—At least one interface of a backbone router must reside in the backbone area. All ABRs and internal routers in area 0 are backbone routers.

• Autonomous System Boundary Router (ASBR)—A router exchanging routing information with another AS is an ASBR, which may not reside on the border of the AS. It can be an internal router or an ABR.

OSPF prioritize routes into the following levels:

• Intra-area route

• Inter-area route

• Type-1 external route

• Type-2 external route

The intra-area and inter-area routes describe the network topology of the AS. The external routes describe routes to external ASs.

OSPF classifies external routes as Type-1 or Type-2. A Type-1 external route has high credibility. The cost from a router to the destination of the Type-1 external route = the cost from the router to the corresponding ASBR + the cost from the ASBR to the destination of the external route.

A Type-2 external route has low credibility, so OSPF considers the cost from the ASBR to the destination of the Type-2 external route is much greater than the cost from the ASBR to an OSPF internal router. The cost from the internal router to the destination of the Type-2 external route = the cost from the ASBR to the destination of the Type-2 external route. If two routes to the same destination have the same cost, OSPF takes the cost from the router to the ASBR into consideration to determine the best route.